Advanced semiconductor devices, such as highly dense dynamic random access memories ("DRAMs"), impose severe restrictions on the times, temperatures and atmospheres of all thermal process steps. This particularly the case following the source-drain implantation. BPSG reflow and source-drain implant activation are two of the process steps that contribute significantly towards enhanced overall thermal budget during the fabrication of DRAM devices.
Borophosphosilicate glass ("BPSG") films or layers are especially important in the step of planarizing advanced DRAM devices having increased stacked capacitor heights and integrated densities. As the formation and subsequent reflow of BPSG films are of critical importance, efforts have been made to investigate its mechanical, electrical, and structural properties. To assure continuity of metal lines over steps at a reduced thermal budget, it is essential to obtain superior quality BPSG films.
Reflow properties at lower thermal budgets can be enhanced by employing a higher concentration of boron in the BPSG layer. However, merging the step of activating source-drain implants with the reflow thermal cycle step produces higher device speeds. A trade-off between the overall thermal budget required for reflow and activation and lower boron concentration can eliminate the problem of dopant out diffusion due to higher boron and phosphorus concentration, as well as provide higher activation of the source-drain regions.
For submicron device fabrication, the use of short time rapid thermal processing ("RTP") cycles at high temperatures is becoming more attractive for shallower junctions and lower diffusion of dopants. Even at short time RTP reflow cycles, it is crucial to attain good conformality by using an optimum process window to anneal out the effects of moisture, aging, and water absorption in the BPSG film which may result in void formation and segregation.